The authors modeled the effects of variables including the start time of a vaccination campaign, number of vaccine doses administered per week and allocation by age group, clinical attack rate, case hospitalization ratio, and case fatality ratio, on an influenza pandemic. The timing of the start of the vaccination campaign relative to the start of the pandemic had the greatest effect, with 141,000 to 2.2 million hospitalizations and 11,000 to 281,000 deaths prevented when the vaccination program started prior to the beginning of the pandemic.